T he liver plays a key regulatory role in cholesterol metabolism. Two proteins are central in this role; the LDL receptor and 3-hydroxy-3-methylglutaryl CoA reductase (HMG CoA reductase), the rate-limiting enzyme in cholesterol biosynthesis. In the current investigation, we have used a sensitive nonradioactive method to study the regulation of LDL receptor and HMG CoA reductase mRNA levels in liver biopsy samples and freshly isolated mononuclear leukocytes from 13 patients who underwent cholecystectomy for gallstones. mRNA copy numbers were determined by PCR amplification of reverse-transcribed RNA using synthetic RNA as an internal standard. Incorporation of digoxigenin-11-dUTP during amplification allowed direct detection and quantitation of mRNA levels by chemiluminescence. These experiments showed that the average number of LDL receptor mRNA molecules in liver (21 +/- 3 x 10(4)/micrograms of RNA) and mononuclear leukocytes (24 +/- 3 x 10(4)/micrograms of RNA) are indistinguishable, whereas the number of HMG CoA reductase molecules in liver (107 +/- 15 x 10(4)/micrograms of RNA) is smaller than that in mononuclear leukocytes (158 +/- 21 x 10(4)/micrograms of RNA, P < 0.05). These numbers correspond to an average of 1-6 copies of LDL receptor mRNA and 5-42 copies of HMG CoA reductase mRNA per cell. There was a significant correlation between the numbers of LDL receptor (P = 0.0005) and HMG CoA reductase (P = 0.003) mRNA molecules in liver and mononuclear leukocytes. Furthermore, the numbers of copies of HMG CoA reductase and LDL receptor mRNA were correlated with each other in both liver (P = 0.02) and mononuclear leukocytes (P = 0.01), consistent with coordinate regulation. These data demonstrate that the mechanisms which regulate mRNA levels in liver and mononuclear cells are similar and suggest that freshly isolated mononuclear cells can be used to predict HMG CoA reductase and LDL receptor mRNA levels in liver.